Long range observation of objects and targets has been the topic of military and civilian system development for many years, for applications ranging from weapon targeting to aircraft/spacecraft flight control/safety operations, including the detection of wires, obstacles and other hazards for helicopter and other manned air vehicles as well as unmanned air vehicle (UAV) operation. The goal of such systems typically involves detection, recognition, and identification of objects or targets at ranges which are sufficiently long such that action can be taken based on the observation in a timely manner to achieve a desired goal—e.g., control aircraft flight along a safe path, direct munitions onto a hostile vehicle. Key attributes of such systems generally include the following: operation under all target/environment conditions—including total darkness and near-zero thermal contrast; operation in the presence of obscurants (e.g., fog, smoke, rain, haze); and operation over long ranges to provide sufficient time for operators to react in a timely manner to achieve their missions before disruption is caused by the object under observation.
A broad range of system and component technologies have been developed to address long range target identification, including passive, visible and near-infrared wavelength sensors; active, visible and near-infrared wavelength sensors; and passive mid-wave infrared (MWIR) and long-wave infrared (LWIR) sensors. In the case of passive, visible and near-infrared wavelength sensors, using well-developed CCD or CMOS cameras, sometimes in conjunction with special photocathodes in electron-multiplication schemes, this approach has been very successful in providing high resolution imagers for many applications. The short operating wavelengths (typically in small individual spectral bands within the wavelength range from 0.5 to 1.5 micrometers) provide excellent resolution with small diameter optics, and unit prices are low, reflecting the maturity of the technologies. Unfortunately, the usefulness of most of these systems degrades dramatically under conditions of reduced visibility, as occurs in darkness or in the presence of obscurants such as haze, smoke, rain, or fog.
Alternatively, with active, visible and near-infrared wavelength sensors, considerable performance enhancements may be obtained by using a time-gated visible or near-infrared wavelength sensor in conjunction with a short-pulse laser having a short wavelength within these spectral regions. By turning the camera on for only brief periods, corresponding to the particular time interval around which the laser pulse returns from the target, a freeze-frame flash photograph of the object is obtained. The contrast-reducing effects of bulk atmospheric backscatter are reduced dramatically and image clarity is correspondingly enhanced, allowing extended range operation. Observation in total darkness is relatively easily achieved, and penetration of obscurants (e.g., fog, smoke, rain, haze) is somewhat enhanced over purely passive systems by gating out much of the optical noise in the images. However, range limitations due to atmosphere attenuation of the laser wavelengths in strongly obscured environments prevent these systems from effective use in many situations.
In the case of more general, passive MWIR and LWIR sensors, using thermal cameras of varying designs (e.g., scanned single detector or arrays, staring arrays; InSb, HgCdTe, or silicon microbolometer materials), systems in this category can provide capability for viewing objects in total darkness and through most obscurants. The former benefit results from the fact that the targets being viewed inherently emit infrared radiation; the latter benefit derives from the relatively good transmissions of many infrared wavelengths through typical obscurants. Unfortunately, the long wavelengths at which such passive systems typically operate requires optics that scale in volume roughly as the cube of the wavelength. The resultant growth often makes such systems too large or expensive for desired applications. Additionally, the thermal contrast between targets and their backgrounds typically degrades dramatically several times during a normal diurnal cycle, making objects difficult to detect, recognize, and identify.
Unfortunately, none of these systems simultaneously provides all of the features desired in a long-distance imaging system. Therefore, an unsatisfied need exists in the art for improved systems and methods that are capable of achieving long range imaging of targets under day or night conditions in the presence of various obscurants, such as smoke, haze, rain, or fog.